1 Parker SSD Drives - Modular inverters control transfer of hydroelectric power

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Modular inverters control transfer of hydroelectric power

02 December 2014

The regulatory framework is now in place to allow small-scale producers of electricity to export and sell their surplus power via the national grid, but transferring that power to the grid is no trivial task.

Applied Hydropower, a specialist in the design and installation of private hydroelectric plant has developed a solution based on modular inverter drives.

The biggest challenge with transferring energy from a small hydroelectric installation to the national grid is synchronisation: the power being fed to the grid must precisely match the phase and frequency of the grid itself.

Achieving accurate synchronisation by conventional methods, such as controlling the speed of the alternator is, however, by no means straightforward. One of the problems is that the flow of water in a small hydroelectric installation fluctuates, which means that the speed of the turbine it is driving and hence the speed of the alternator also fluctuate.

A second challenge is to have the turbine working at its peak efficiency all the time even though the supply of water flow and pressure varies considerably, requiring the turbine speed to vary with it.

Meanwhile, a third challenge is to provide smooth shock-free starting for the turbine and alternator. Simply allowing the full force of the water to reach stationary turbine blades would impose large mechanical shock loads.

To address these issues when refurbishing the hydroelectric installation at Vivod Hall, a stately home in North Wales, engineers from Applied Hydropower decided to use Parker’s AC890 modular inverter drives.

The drives are not used in the usual way to control the speed of a motor; instead, two bi-directional inverter modules are connected back-to-back via their dc buses. In essence, one of the inverters is connected to the hydroelectric installation’s generator, the other to the national grid. The generators used are induction generators.

With this arrangement, the speed of the induction generator may be varied, but the supply from the hydroelectric installation is always synchronised with the grid. The peak efficiency of the system may be set for different water conditions by measuring the induction generator slip between the frequency of the supply and the induction generator speed.

Automatically raising and lowering the frequency by small increments enables the system to set its running frequency in order to find the most advantageous operating frequency.

This arrangement also has other benefits; as it is electrically bi-directional, it is possible to draw power from the grid to drive the induction generator as a motor. This means that it can be run up to speed at system start up, bringing the turbine up to speed prior to turning on the water, thereby eliminating mechanical shock problems.

The new hydroelectric installation at Vivod Hall has two turbines, one of which is used to supply the 8kW needed to drive a heat pump and to provide hot water and lighting for the house.

The second and larger of the two, which is rated at 22kW, is configured with the Parker drives and has the capability to export all its power. In this case, it proved possible to use standard totally enclosed six-pole motors as induction generators operating at frequencies from 40 to 70Hz and coupled directly to the turbine.

Parker built all of the system in one pane,l including the metering, G59 protection system, sequencing and control and modems for remote operations, thus avoiding the multiple suppliers and skills for electrical assembly and servicing that Applied Hydropower would otherwise have needed.

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